Labeling SNAP-tag Fusion Proteins with BG-substrates (S9148)

Introduction

Once the building block has been converted into a BG-substrate it can be used to label SNAP-tag fusion proteins. These can be labeled in cell lysates or as purified protein.

For labeling, we recommend using a 1.5 fold excess of substrate to SNAP-tag fusion protein and incubation for 1 hour at room temperature. Typical concentrations are 30 µM substrate and 20 µM SNAP-tag fusion protein. The labeling incubation can be followed by a separation step such as dialysis or spin column separation to remove unreacted substrate, if desired.

Notes:

Unlabeled protein samples should be stored at –20°C, or
at –80°C for long-term storage. Many proteins benefit from the addition of
glycerol for frozen storage, typically 20% v/v. Handling at temperatures above
0°C should be minimized by thawing the unlabeled protein samples shortly before
use, and keeping them on ice until just before the labeling reaction.

We
recommend the routine addition of 1 mM DTT to all buffers used for handling,
labeling and storage of SNAP-tag fusion proteins. The stability of the SNAP-tag
is improved in the presence of reducing agents; however it can also be labeled
in their absence.

If a particular fusion protein requires buffers
without reducing agents, minimize all handling steps of the protein above 4°C
prior to the labeling reaction.

The SNAP-tag labeling reaction works
well between pH 5.0 and pH 10.0 The salt concentration may need to be optimized
for your particular fusion protein, although the SNAP-tag labeling reaction has
been shown to work at a broad range of ionic strengths (NaCl concentrations
between 15 mM and 1 M). If the buffer conditions used for labeling lead to
insolubility of your protein, we recommend testing a range of pH and ionic
strengths. For a range of fusion proteins we have found ionic strengths for
monovalent salts (e.g. sodium chloride) from 50 mM to 250 mM
helpful.

Where stickiness of the fusion protein is a problem we
recommend adding Tween 20 at a final concen-tration of 0.05% to 0.1%. At this
concentration Tween 20 does not affect the performance of the SNAP-tag. Ionic
detergents (e.g. SDS) should be avoided.

The maximum degree of labeling
possible with the SNAP-tag is one molecule of label per molecule of fusion
protein.

Storage and Handling of Labeled SNAP-tag Fusion
Proteins
Labeled SNAP-tag fusion proteins should be stored at
concentrations of 1 mg/ml or less at 4°C. Sodium azide may be added to 2 mM
final concentration to prevent bacterial growth. Under these conditions the
labeled protein should be stable at 2–6°C for several months. If freezing is
required, it may be best to shock freeze by rapidly cooling the sample to a
temperature significantly below 0°C in the presence of 20% (v/v)
glycerol.

Labeled protein is linked to the SNAP-tag label by a covalent
bond. Therefore the conjugates are highly stable. DTT or other reducing agents
are not required in the storage buffers. Since the fusion protein is covalently
labeled, labeled protein may be detected after harsh or even denaturing
treatments, such as chromatographic or electrophoretic separation including
SDS-PAGE.

Avoid repeated freezing and thawing. Protect from light if the
protein is labeled with a fluorophore.

Troubleshooting

Labeling Reaction
If exhaustive labeling of a protein sample is not achieved using the recommended conditions, try the following protocol modifications: Double the incubation time to two hours total at 25°C or to 24 hours at 4°C; or double the ratio of label to protein in the labeling reaction. Both approaches may be combined. If you still have poor labeling results, we recommend checking the activity of the SNAP-tag.

If your fusion protein is particularly sensitive to degradation or to loss of activity, you can try reducing the labeling time or decreasing the labeling temperature. If you label at 4°C we recommend overnight incubation.

Inefficient Labeling
DMSO at greater than 1% of the volume of protein solution may slow the reaction rate. Lower substrate concentrations also slow the reaction rate. Increasing the reaction time may be helpful in these cases.

Activity of the SNAP-tag can sometimes be partially or completely lost. This can be caused by extended storage of non-reacted SNAP-tag fusion proteins at 4°C or greater. The sensitivity of the SNAP-tag to inactivation is also significantly increased if no reducing agent is added.

If you believe that the activity of the SNAP-tag is affected, we recommend analyzing a small fraction of it on a PAGE gel using the SNAP-Vista Green (NEB #S9147) to confirm that the SNAP-tag is active.

If you encounter problems with the activity we recommend thawing another sample of your protein or reexpressing and repurifying the SNAP-tag fusion protein following the advice given in the SNAP-tag plasmid instructions.

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